1. Field of the Invention
This invention relates to a mass flowmeter that operates by the Coriolis principle and incorporates an essentially straight Coriolis measuring tube through which flows a process medium, two essentially straight connecting pipes conducting said flowing medium, at least one oscillator associated with and stimulating the Coriolis measuring tube, at least one measuring sensor associated with the Coriolis measuring tube for detecting Coriolis forces and/or oscillations generated by Coriolis forces, and a compensating cylinder, wherein said Coriolis measuring tube is housed within said compensating cylinder, each of the two ends of the compensating cylinder connects in mechanically fixed fashion to an end section of the Coriolis measuring tube, each of the two end sections of the Coriolis measuring tube transitions into a connecting pipe, said connecting pipes are positioned outside the compensating cylinder, and by way of mounting adapters provided at the external ends of the connecting pipes, the mass flowmeter can be installed in a pipeline system. An example of this type of mass flowmeter has been described in the technical publication “atp—Automatisierungstechnische Praxis” (Applied Automation Technology) No. 40 (1998), pp. 24-29.
As stated above, the mass flowmeter according to this invention incorporates at least one oscillator “associated with” the Coriolis measuring tube, and at least one measuring sensor likewise “associated with” the Coriolis measuring tube. The oscillator or at least part of the oscillator, and the measuring sensor, or at least part of the measuring sensor are usually connected to the Coriolis measuring tube. However, because that is not an absolute requirement, the term “associated with” has been chosen in lieu of “connected”.
In the case of mass flowmeters employing the Coriolis principle, a fundamental differentiation is made between configurations employing an at least essentially linear i.e. straight, Coriolis measuring tube, and those in which the Coriolis measuring tube is looped. Among these mass flowmeters one also distinguishes between designs employing only one Coriolis measuring tube and those equipped with two Coriolis measuring tubes, where two Coriolis measuring tubes are used. These may be positioned either in tandem or side-by-side for a straight-line or parallel flow.
In recent times, there has been an increasing preference for mass flowmeters employing only one, essentially straight Coriolis measuring tube. Coriolis-type mass flowmeters with only one straight Coriolis measuring tube offer considerable advantages over mass flowmeters with two straight Coriolis measuring tubes or one looped Coriolis measuring tube: Their main advantage over mass flowmeters using two straight Coriolis measuring tubes lies in the fact that they do not require any of the flow dividers and flow recombiners or junctions that are needed for dual Coriolis measuring tubes. Compared to mass flowmeters with one looped Coriolis measuring tube or even two looped Coriolis measuring tubes, their primary advantage is seen in the fact that a straight Coriolis measuring tube is easier to produce than a looped Coriolis measuring tube, there is less pressure drop in a straight Coriolis measuring tube than in a looped Coriolis measuring tube, and a straight Coriolis measuring tube can be cleaned more easily than a looped Coriolis measuring tube.
On the other hand, Coriolis-type mass flowmeters with a straight Coriolis measuring tube have one intrinsic drawback insofar as, in mass flowmeters employing the Coriolis principle, it is necessary for at least one oscillator to cause the Coriolis measuring tube or tubes to oscillate and that, ultimately, the Coriolis measuring tube or tubes oscillate(s) as a function of the Coriolis forces or resulting Coriolis oscillations while a process medium flows through the Coriolis measuring tube or tubes. In the case of mass flowmeters using two straight Coriolis measuring tubes or a looped Coriolis measuring tube or two looped Coriolis measuring tubes, these Coriolis measuring tubes or the oscilating sections of the looped Coriolis tubes are identical in design, and they are positioned and stimulated in such fashion that they oscillate against each other. As a result, the overall oscillating system has no oscillatory effect on its surroundings. The center of mass of that system remains stationary, which compensates for any impinging forces. It follows that this type of mass flowmeter does not transfer any oscillations into the pipeline system in which it is installed and, consequently, there are no retroreflected oscillations from the pipeline that would compromise the measuring accuracy.
Evidently, Coriolis-type mass flowmeters equipped with only one straight Coriolis measuring tube do not offer this functional feature of mutually counter-oscillating Coriolis measuring tubes or sections thereof Therefore, in a mass flowmeter with only a single Coriolis measuring tube, the center of mass does not remain stationary and there is no compensation for impinging forces. As a result, the oscillations are transferred into the pipeline system in which the mass flowmeter is installed from where they can be reflected back.
2. Description of the Prior Art
As a means to at least reduce the above-mentioned problem inherent in Coriolis-type mass flowmeters employing a single Coriolis measuring tube, the mass flowmeter first above described and on which this invention is based can be equipped with a compensating cylinder. Such a provision can significantly lessen the forces that would otherwise bear on the mounting hardware by means of which the mass flowmeter is installed in the pipeline system.
As another possible improvement in a mass flowmeter that incorporates a single straight Coriolis measuring tube in conjunction with a compensating cylinder, DE 197 10 806 A1 proposes to symmetrically mount a symmetrically configured equalizing system on the compensating cylinder. That equalizing system would be so designed as to minimize, preferably close to zero, the oscillating amplitude of the compensating cylinder.
DE 198 40 782 on its part proposes an equalization of both the excitation oscillations and the Coriolis oscillations of the Coriolis measuring tube within the compensating cylinder. Accordingly, neither the stimulation i.e. excitation oscillations nor the Coriolis oscillations would affect the compensating cylinder. In other words, neither the excitation oscillations nor the Coriolis oscillations would cause the compensating cylinder to oscillate, leaving the compensating cylinder in an unaffected, quiescent state. This means that it is possible to further improve a Coriolis-type mass flowmeter employing only one essentially straight Coriolis measuring tube by keeping the center of mass of the overall assembly of the compensating-cylinder components in a stationary state, or in fact by keeping the center of mass of the overall assembly composed of the Coriolis measuring tube, the oscillator or oscillators and the measuring sensor or sensors in a stationary state. If there are other components contained within the compensating cylinder, it is of course necessary to include them in this equation.
Moreover, EP 0 759 542 A1 and EP 0 831 306 A1 describe mass flowmeters in which the compensating cylinder is provided with weights designed to match its intrinsic natural frequency with the resonant frequency of the Coriolis measuring tube. U.S. Pat. No. 5,796,010 on its part describes an additional mass that is mounted on the compensating cylinder. That mass is intended to reduce the resonant frequency of the mounting hardware for the Coriolis measuring tube, which includes the compensating cylinder. Two alternative methods are offered for mounting the additional mass, one providing for it to be placed in the center of the compensating cylinder while the other involves the attachment of additional elements of mass at the ends of the compensating cylinder.
Finally, DE 199 08 072 A1 describes a mass flowmeter employing the Coriolis principle and incorporating one single, straight Coriolis measuring tube, where for the mass equalization of the excitation oscillation, a first balancing mass is provided and connected to the compensating cylinder along the central plane extending in a perpendicular direction relative to the longitudinal axis of the latter, while for the mass equalization of the Coriolis oscillation, a second balancing mass and a third balancing mass are provided in the form of end sections of the compensating cylinder. This approach is intended to at least largely equalize the mass of the oscillation-capable system consisting of the Coriolis measuring tube and the compensating cylinder with regard to both the excitation oscillations of the Coriolis measuring tube and the Coriolis oscillations of the Coriolis measuring tube.